My buddy Bill told me that eworkmarkets.com(now defunct) was what he used, until they folded. They charged $$$ and left a lot of people unpaid, no warning - in 2007. He also had a cool idea - an app that emails you when certain jobs match across several sites. Some might have this function already. Also - if you wrote a very niche app but wanted to know when/if someone was looking for that type of app. There are sites that you can sign up for for "reputation" reviews
like rapleaf and spock. So if some client is on that, and has good feedback/reputation, you have more confidence in them. You can also use these sites: programmermeetdesigner.com
, xing, linkedin, rentacoder, elance. The problem with a lot of these job sites is that they often have mostly low $ stuff.